The invention relates to a method and device for determining deviation of a nominally planar surface of a thin object, relative to a reference plane.
Such a method and such a device have for years been used by the applicants and are therefore known. The known solutions are provided and arranged for measuring wafers, so that an object to be measured is formed by a wafer. With the known solutions the support members and, consequently, the support positions are arranged in a more or less arbitrary position relative to each other. The main reason for this lies in the fact that the known solutions are provided for determining deviations of a nominally planar upper surface of a thin and, in essence, circular disk-shaped object, that is, a wafer, relative to a reference plane for which fact the wafer is thin and has a thickness, for example, in the region of 200 xcexcm or more, but has nevertheless a thickness, that is the thickness of about 200 xcexcm or more, with which practically no parasitic warpage occurs that can be attributed to the influences of gravity on the wafer. The known solutions are unsuitable for thinner wafers, because parasitic warpage occurs owing to gravity, which warpage leads to additional deviations of the nominally planar surfaces of such a wafer relative to a reference plane, which leads to erroneous measuring results with the known solutions. Since in practice, however, there is a tendency to come to ever thinner wafers, there is a problem that for such thinner wafers no satisfactory measuring results can be achieved with the known solutions with regard to finding deviations in a surface of a wafer relative to a reference plane.
With regard to a known method and a known device, also the publication of patent document DE 42 31 205 A1 may be referred to in this respect. This document has also disclosed the use of three support members lying in the three corners of an equilateral triangle. However, no information is given about the positions of the three support members relative to the object to be measured, that is to a composite body.
To achieve the above-defined object of the invention, the method includes: determining deviations of a nominally planar surface of a thin object relative to a reference plane, wherein the object is subject to warpage related to gravity, the method comprising the step of supporting the object by three support positions, wherein the three support positions are in a constant position relative to the reference plane; and detecting deviations of the nominally planar surface relative to the reference plane, wherein each support position is located underneath the center of gravity of a circle sector that stretches out over an angular region of 120xc2x0 of the object.
Providing the characteristic features according to the invention achieves in an extremely simple manner and without additional means that the influence of gravity on the results of the measurement, which can be determined when deviations from a nominally planar surface of a thin and, in essence, circular disk-shaped object are measured relative to a reference plane, can be practically ignored because, based on the invented arrangement of the support members and as a result of the support positions of the object, practically no parasitic warp of the object owing to the influences of gravity occurs.
With a device according to the invention it has proved to be highly advantageous when, the device is adapted very simply to different sizes, thus to different diameters of circular disk-shaped objects.
The above-defined aspects and further aspects of the invention will be evident from the example of embodiment to be described hereinafter and will be explained with reference to this example of embodiment.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.